#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

int main() {
    int a, flg = 1;
    double sum = 0, tmp = 0;
    while (scanf("%d", &a) != EOF)
    {
        for (int i = 1; i <= a; i++)
        {
            tmp += flg * (2 * i - 1);
            sum += 1.0 / tmp;
            flg = -flg;
        }
        printf("%.3f", sum);
    }
    return 0;
}